Chitin shows a random distribution in the cell wall of several Saccharomyces cerevisiae mutants blocked in the cell cycle at a nonpermissive temperature. Chitin synthetase zymogen is transformed into active enzyme in proteinase B-defective yeast strains treated with digitonin. The activation process is inhibited by certain protease inhibitors and by chelating agents,. Calcium ions reverse the effect of the latter. A chitinase was purified from the growth medium of Serratia marcescens and used for the determination of chitin.